Irritable bowel syndrome (IBS) is a disorder that shows dysfunction of lower bowel such as abnormal defecation (diarrhea or constipation) and concomitant abdominal symptoms of abdominal pain and discomfort lasting for several months.
The pathophysiology of irritable bowel syndrome is still unclear, but it is now that mental stress is closely involved in the development of symptom, and a high coincidence rate of mental disorder such as depression and hysteria has been made known. Sleep disorder is often observed in patients with irritable bowel syndrome, and it is reported recently that sleep disorder in patients with irritable bowel syndrome correlates with the degree of abdominal symptoms (see non-patent documents 1.
In addition, there is a report that, compared to healthy individuals and patients with irritable bowel syndrome having no depressive symptoms, patients with irritable bowel syndrome having depressive symptoms show more severe abdominal symptoms and sleep disorders (see non-patent document 2).
Further, there is a report that melatonin, known as a hormone regulating sleep-wake cycle, improves significantly the abdominal symptoms such as abdominal pain, abdominal bloating and sense of urgency for defecation in female patients suffering from irritable bowel syndrome (see non-patent document 3).
Furthermore, there is a report that melatonin shows an inhibitory action on partial restraint stress-induced defecation which is an experimental model based on defecation abnormality for irritable bowel syndrome (see non-patent document 4).
In addition, patent document 1 discloses that (S)—N-[2-(1,6,7,8-tetrahydro-2H-indeno[5,4-b]furan-8-yl)ethyl]propionamide (general name: Ramelteon) has a melatonin receptor MT1/MT2 agonistic action.    [patent document 1] U.S. Pat. No. 6,034,239    [non-patent document 1] Monica Jarrett et. al., Digestive Diseases and Sciences, Vol. 45, No. 5 (May 2000), pp. 952-959    [non-patent document 2] Jennifer J. T. Robert et. al., Digestive Diseases and Sciences, Vol. 49, Nos. 7/8 (August 2004), pp. 1250-1258    [non-patent document 3] W. Z. Lu et. al., Aliment Pharmacol Ther 2005; 22: pp. 927-934    [non-patent document 4] G. S. Song et. al., Neurogastroenterol Motil 2005; 17: pp. 744-750